Fix multi-project port identity: validate persisted selection on restore

Two-layer defense against port swapping when multiple Unity projects restart:

Layer 1 (index.js): ensureInstanceDiscovery() now calls validateSelectedInstance()
when restoring from persisted state. Detects port swaps (different project on
saved port) and re-discovers instances by projectPath. Clears stale selections
if the project is no longer running.

Layer 2 (instance-discovery.js): New validateSelectedInstance() function that
pings the saved port, verifies projectPath identity, and re-selects by matching
projectPath across all discovered instances if a swap is detected.

Bump version to 2.22.0